Printed electronics constitute a class of electronic devices that are made by printing or etching conductive materials on a substrate. Printed circuit boards (PCBs) are typically made on rigid fiberglass sheets, however plastic and paper substrates have emerged as alternative materials that are flexible, lightweight, and disposable. To date, metallic and carbon-based features have been patterned by metal evaporation, sputter coating, inkjet printing, screen printing, or roll-to-roll transfer. Most methods either require sophisticated equipment or employ dilute inks that wet and spread on smooth plastics (resulting in poor resolution) or readily permeate paper (resulting in poor conductivity). There is a need for facile, inexpensive printing methods that yield conductive and resistive traces on flexible substrates. In this thesis, we develop two platforms for patterning conductive silver and resistive carbon inks for flexible electronics: (1) rollerball pen printing and (2) direct ink writing (DIW).
Conductive silver and resistive carbon particle-based inks are created for direct writing of flexible electronics. Specifically, a new silver ink is synthesized that enables printing of high conductivity features, approximately 1% of bulk silver, at ambient conditions. A complementary carbon nanoparticle ink is formulated that exhibits an electrical conductivity that is six orders of magnitude lower than the silver ink. The rheological properties of these inks can be widely tuned to facilitate uniform flow through commercial rollerball pens or printing of continuous filaments via direct writing. Both the solids loading and hydroxyethyl cellulose (HEC) content within these inks strongly influence their rheological and printing behavior. Importantly, by blending these two inks, we can form composite inks, whose electrical resistivity can be systematically varied over several orders of magnitude between 2x10^-4 Ohm-cm (pure silver ink) to 200 Ohm-cm (pure carbon ink).
Pen-on-Paper electronics offers an inexpensive and intuitive approach to creating paper electronic devices. By filling commercial rollerball pens with conductive silver and resistive carbon inks, we can pattern functional features on low-cost substrates. Direct observation of the ink transfer during printing revealed that conductive silver dispensed from rollerball pen inks undergo a well known “printer’s instability”, in which the trailing edge of the ink breaks up into two fin-like meniscuses, above a critical printing speed. We find that a balance between viscous and surface forces determines the meniscus shape and corresponding printed line morphology; specifically, this transition occurs at Ca~0.1. The influence of the paper substrate on printed line morphology is also investigated. Semi-gloss photo paper provides a unique writing surface, in which solvent is rapidly wicked through a microporous plastic film, pinning the edges of the printed ink traces and preventing ink spreading. In this study, semi-gloss photo paper is used as a model surface for ink printing dynamics. We demonstrate multiple examples of Pen-on-Paper electronic devices, including electronic art, analog filters, tactile and visual user interfaces, and a paper Arduino circuit board (i.e., Paperduino) printed using an inexpensive desktop pen plotter.
To create novel photovoltaic devices, silver interconnects and busbars are printed onto glass and polyimide substrates (6” diameter) that are populated with an array of InGaP micro solar cells by direct writing a concentrated silver ink. We introduce an optical profilometry method to map the topography of the complex surface and dynamically adjust the height of the printing nozzle to within 15 +/- 5 microns of the surface during printing. The flexible polyimide PV devices are thermally annealed at 175C for 30 minutes, yielding silver traces with conductivity 2.5x10^-5 Ohm-cm. The PV cell current-voltage (I-V) response is analogous to measurements performed on cells wired with evaporated gold interconnects, confirming the quality of our printed silver features. This research opens new avenues for patterning low-cost, flexible electronics for applications ranging from science education to advanced technologies.